This invention relates to reciprocating engines such as gasoline or diesel internal combustion engines, as well as to Stirling or other external combustion engines. The term engine as used herein is not limited only to prime movers or power sources, but can also be applied to other reciprocating devices such as piston type pumps and compressors.
In a typical reciprocating engine, a piston oscillates within a cylinder, and defines within the cylinder a chamber of variable volume. This chamber has minimum and maximum volumes, respectively, at piston positions corresponding to top dead center, (where the piston is closest to the cylinder head), and to bottom dead center, (where the piston is closest to the crank or other drive linkage).
In a conventional internal combustion engine, a combustion or power stroke normally occurs at the phase of the cycle in which the piston travels from top dead center to bottom dead center. Conversely, a compression stroke occurs on the phase in which the piston travels from bottom dead center to top dead center. In an internal combustion engine, the maximum forces occurring on the power stroke far exceed the maximum forces sustained on the compression stroke. In order to provide more stable and smoother operation, and to reduce internal forces on the drive linkages, it is desirable for the power stroke to occupy less than 180.degree. of crank travel. This provides a greater mechanical advantage to the expanding gas within the chamber because of the optimization of rotation angle to the power stroke.
There have been several previous attempts to alter the power stroke by employing toggle linkage systems that connect a piston to a pair of parallel, counter-rotating cranks. Two such systems are described, for example, in U.S. Pat. Nos. 1,585,796 of May 25, 1926, and 2,392,921, of Jan. 15, 1946.
In these previous patented systems, the piston reciprocates on a line perpendicular to a plane defined by the axes of the two cranks. The length of the connecting rods that connect the piston to the crank is proportioned, with respect to the length of the crank and the distance between the two crank shafts, so that the angle between the connecting rods is less than 90.degree. when the piston is at bottom dead center.
While these previous attempts reduced the crankshaft rotational angle that corresponded to a power stroke, these particular twin-crank drive mechanisms have not proved to be entirely practicable. One reason for this is that there is a high compressive force on the drive linkage, that is, the piston pushes against the connecting rod. The highly elevated compressive forces, especially after combustion on the power stroke, tend to buckle or break the connecting rods, and can also cause heavy wear on the wrist pins where the connecting rods are journalled to the cranks.
An improved drive linkage for a reciprocating engine is described in U.S. Pat. No. 4,898,041, of Feb. 6, 1990. In that engine, each piston travels in its associated cylinder between top dead center (TDC) and bottom dead center (BDC). A pair of counter-rotating cranks have crank arms joined by respective connecting rods to their associated pistons. The two cranks define a crank plane perpendicular to the piston travel, with the rotary motion of the cranks for piston travel from TDC to BDC being less than 180.degree. (e.g. 130.degree.) and from BDC to TDC being more than 180.degree. (e.g., 230.degree.). The piston rod extends from the piston to the connecting rods between and below the crank axes so that the lower end of the piston rod is below the crank plane for all aspects of piston travel. With this engine there is substantially reduced dwell just after top dead center, because for a given rotation angle of the cranks in the vicinity of top dead center, the piston descends from TDC substantially twice as far as the piston ascends to reach TDC. This engine operates with increased torque and power, and greater fuel economy, as well as with the ability to produce significant torque at low speeds (i.e., below 2000 rpm).
Unfortunately, because of the uneven motion of the pistons relative to crank rotation, even where crank rotation speed is uniform, the piston motion is difficult to balance. This is the case even for multiple-piston engines. In fact, in a four-cylinder version of this engine, there is a brief interval in each rotation of the cranks in which all four pistons are ascending. While this does not cause severe problems at low speeds, whenever the speed exceeds about 2000 rpm, the engine vibrations become significant. Counterweights on the cranks or on the flywheel will not balance the uneven piston motion.